Return mechanism for fail safe instruments



July 10, 1962 J. w. ANGUS 3,043,580

RETURN MECHANISM FOR FAIL SAFE INSTRUMENTS Original Filed Sept. 28. 19564 Sheets-Sheet 1 July 10, 1962 J. w. ANGUS RETURN MECHANISM FOR FAILSAFE INSTRUMENTS 4 Sheets-Sheet 2 Original Filed Sept. 28. 1956 NGPx WW8v mw July 10, 1962 J. w. ANGUS RETURN MECHANISM FOR FAIL SAFEINSTRUMENTS 4 Sheets-Sheet 3 Original Filed Sept. 28. 1956 was; we 04mtown/rap /02 M am FIG. 6

$717716 "list/RE (M05074? 78 {CHI July 10, 1962 J. w. ANGUS RETURNMECHANISM FOR FAIL SAFE INSTRUMENTS 4 Sheets-Sheet 4 Original FiledSept. 28. 1956 \& x M M w a I IIIIJL a T; L L

INVENTOR. Jfl/WEJ 1% 4 1 47:

55 Ja /[Iv United States Patent Ofiice 3,043,580 Patented July 10, 19628 Claims. (Cl. 267-1) This invention relates to a spring returnmechanism for biasing a rotatable mechanism toward a predeterminedangular position and is a divisional application of my copendingapplication, Serial No. 612,780, filed September 28, 1956, entitledAircraft Instrument--Remote Control-- Fail Safe, and assigned to theassignee of the present invention.

While mechanically actuated aircraft instruments such as altimeters havebeen refined to provide very accurate indications, they cannot be as apractical matter inherently include compensation for all possiblefactors. Such compensation and correction may be achieved by anelectrical system including, where desired, computing devices as well asinformation gathering and transmitting devices which may integrateinformation. Where the indication is transmitted to the dialelectrically, any failure in the electrical system will make the deviceinoperative.

Therefore, a unit was provided of a substantially standard mechanicallyor otherwise operated aircraft instrument, suchas an aneroid altimeter,together with electrical means for operating the unit as a whole ineither of two ways: (1) to correct or adjust the unit in accordance withelectrically tnansmitted information or (2) to operate the unit as awhole in response to electrically transmitt-ed information. In the firstcase, the electrically transmitted information is used to correct themechanically achieved indication. In the second case, the electricallytransmitted information operates the unit as a whole to achieve anindication. In either case, in the event of failure of the transmittingelectrical network, the mechanical elements in the unit Will take overto perform an indicating job. In this way, the mechanical elements havealso the stand-by function of providing an indication in the event offailure of the electrical system.

In one embodiment of my invention, a mechanical actuating system withina rotatable frame is utilized to operate a pointer with respect to astationary indicating dial, and an electrical system is provided forrotating the frame and all internal elements including the pointer withrespect to the indicating dial.

Hence, a mechanical reading is obtained by the rotation of the pointerwith respect to its frame while a correction of the mechanical readingis obtained by rotating the frame and its associated pointer by means ofelectrically actuated devices.

The frame may be then further constructed so as to be biased by thespring return means of the invention to a zero correction position orposition from which the housing is rotated by the electrically actuatedcorrection.

Hence, it may be seen that with my novel instrument, the pilot of anaircraft will still have the benefit of, for instance, the mechanicalaltitude reading, although the electrical correction system whichprovides a highly accurate altitude reading when operative is disabled.

Thus, a mechanical instrument may be corrected for instrument errors byan electrically operated instrument error correction system whichchanges the frame position in accordance with the mechanical reading ofthe instrument.

Similarly, static system errors which are functions of mach number andangle of attack may be corrected.

While the electrically actuated portion of my novel system may beutilized to add or subtract from the mechanical reading, the electricalportion may alternatively be a central air data computer whichcompletely computes the accurate reading for the instrument, and this.

value is servoed to the pointer by 'a motor which rotates the fname ofthe instrument, the pointer position being fed back by a Synchrotelattached to it, the frame operating merely as a link in the systemduring electrical operation. When the Synchrotel performs a plurality ofcomplete rotations over the full range of the instrument, the instrumentwithin the frame is needed further to obtain a coarse positioning of thepointer within a 180 rotation of the Synchrotel attached to the pointer,the air data computer providing the accurate value within this coarserange.

Hence, while the electrical system is operative, the pointer readingwith respect to its indicating dial will be the most accurate valuegiven by the electrical system, whereas the less accurate mechanicalreading is available in the event of a failure of the electrical systemas was true of the above-mentioned embodiment in which the electricalsystem operated as a correcting means.

Accordingly, a primary object of my invention is to provide aninstrument system having the high accuracy of an electrical measuringdevice while still affording, the reading of a mechanical device in theevent of a failure in the electrical system.

Still another object of my invention is to provide a mechanicalindicating instrument whose reading may be modified in accordance withan electrically calculated error, and is biased to a Zero uncorrectedposition by novel biasing means.

Still a further object of my invention is to provide a mechanicalmeasuring instrument whose value may be modified in accordance with anelectrically computed value wherein spring biasing means are provided toreturn the instrument to its uncorrected value responsive to failure ofelectrical power.

These and other objects of my invention will now be come apparent fromthe following description when taken in connection with the drawings inwhich:

FIGURE 1 is an exploded perspective view of an indicating deviceconstructed in accordance with my novel invention.

FIGURE 2 is a schematic diagram illustrating the operation of my novelsystem.

FIGURE 3 is a schematic diagnam similar to FIGURE 2 and illustrates asecond embodiment of my novel invention.

FIGURE 4 shows an exploded perspective view of my novel spring returnmechanism.

FIGURE 5 shows a side View of the assembled spring return mechanism ofFIGURE 4.

FIGURE 6 shows a modification of the embodiment of FIGURE 2.

Referring now to FIGURE 1, it is seen that the mechanical portion of asystem is comprised of the two aneroid elements 20 and 21 which areconnected in parallelto the rocking shafts 22 and 23, respectively,through arms which engage the bimetal compensators 24 and 25. Thesebimetal compensator-s compensate for the ambient temperature effect onthe altimeter mechanism so as to provide full altitude rangecompensation over a wide range of temperature. The rocking shafts 22 and23 are supported on members 26 and 27, respectively, which are in turnconnected to the ends of the mechanism frame 47 indicated by the dottedlines. Thus, member 26 is connected to the mechanism frame 47 at points28 and 29 while member 27 is connected to the mechanism frame at thepoints 3t and 31.

tion.

7 less operative.

sosaeso a gear associated with the shaft 34. Shaft 34 is then connectedthrough the gear train including gears 35, 36, 37 and 38 to theinstrument pointer 39 which rotates with respect to the indicating dial40.

The shaft 34 is also connected to a drum 41 by means of the gears 42 and43, the drum 41 serving as an indicator for the number of thousands offeet, while the rotation of pointer 39 indicates a one thousand footchange inaltitude per 360 rotation.

In an assembled instrument, the drum 41 would be observable throughaperture 44 in the indicating dial It is to be noted that the mechanismbody or frame 47 which has the ends comprising the plate 45 and gear 46is rotatable with respect to the mechanism shown in FIGURE 1 as being infront of the gear 46. Hence, a rotation of the frame 47 will effect arotation of the pointer 39 which is connected to frame 4'7 with respectto the indicating dial 40.

That is to say, the pointer 39 which, of course, includes indicatingdrum 41 as an essential part thereof is rotatable with respect to thedial in two manners. The first would be a rotation due to rotation ofshaft 34 because of expansion or contraction of aneroid elements and 21and the second by means of a rotation of a complete frame assembly 47.

Two barometric counters 48 and 49 are then provided where drum 48indicates inches of mercury while drum 49 indicates millibars.Barometric counters 48 and 49 are adjustable by the adjustment knob 50which rotates a gear 51 which in turn cooperates with the spiral gear52.. Spiral gear 52 then actuates the gear train including gears 53, 54,55, 56, 57 and 53 where gear 55 actuates the barometric'counter 48 andgear 57 actuates the barometric counters 49 and at the same time rotatesframe 47 the appropriate amount through shaft 64, gear 76, differential67, gears 66 and '65 and finally gears 62 and 46.

The above-described system can be the mechanical altimeter which isutilized in my novel invention. It is to be noted, however, that themechanical altimeter shown in FIGURE 1 is merely illustrative of thetype of mechanism which I can utilize in my novel system.

So as to allow electrical correction of the mechanical reading of the'mechanical instrument described above, the shaft 34 is provided with agear 59 which cooperates with gear 60 of Synchrotel 61.

The operation of Synchrotel 61 as will be shown more fully hereinafteroperates so as to allow certain electrical corrections to proceed, suchas instrument error correc- The electrical correction factor is appliedto the instrument of FIGURE 1 through the gear 46 by means of thecooperating gear 62 which is attached to a sleeve 63 connected to a gear65. The gear 65 is, in turn, connected to a gear 66, a mechanicaldifferential 67, and gears 63, 69, 70, 71 and a control motor 72. Inlike manner, the mechanical differential 67 is connected to a means 73for automatic return of frame'47 to a noncompensated position responsiveto electrical failure and will be more fully described hereinafter inconnection with FIGURES 4 and 5.

During normal operation, with electrical power available to the system,the position of the pointer 39'due to the aneroid elements 20 and 21will be modified by means of the electrical compensating systemoperating through the gear 62 torotate frame 47 and its associatedpointer 39 with respect to the indicating dial 40.

In the event that there is a failure of electrical power, the automaticreturn means 73 will automatically return the frame 47 to itsnon-compensated position so that the complete indicating instrument mayoperate as a mechanical system which is uncompensated but is neverthe-Thus, a failure of electrical power will not completely destroy theinstruments value and while operates at higher accuracy.

It is to be noted that the servomechanism including the motor 72 andsynchro 74 is connected through the wall 75 of the instrument case toprevent conduction of heat to the sensitive mechanism Within the frame47. It is to be further noted that both the Synchrotel 61 and drum 41Which are geared to the pointer shaft 34 are low inertia, low frictionsystems, and their effect on the instrument reading when operated as amechanical system is negligible.

As has been previously mentioned, provisions are made to adjust thebarometric reading of the barometric counters 48 and 49. This adjustmentis also made in the case of FIGURE 1 to affect the position of pointer39 since a rotation of adjustment knob 50 will rotate gear 76 which inturn will rotate gears 66, 65, 62 and gear 49 of the frame 47.

It is important to note that the barometric adjustment which isdesirable to adjust the altimeter or instrument to the barometriccondition of the area in which the aircraft is located or will belocated upon landing rotates frame 47 through the differential 67.

Hence, the barometric adjustment and the adjustment due to theelectrical system may both be made at the gear 65 independently of oneanother. Therefore, the electrical correction system will correct theinstrument reading which has already been independently corrected forlocal barometric conditions.

The return mechanism 73 of my novel invention may be of a springactuated type as shown in FIGURES 4 and 5 which shows the synchro 74 ofFIGURE 1 and its output gear 74a attached to the rotor shaft 103. Therotor shaft 103 is connected to the gear 74a. A disk 104 isconcentrically mounted with respect to shaft 103 and is freely rotatablewith respect thereto.

Disk 104 carries springs 105 and 106 thereon, the interior portions ofthe springs being fastened to the disk while the outer portion of spring105 is anchored to the synchro gear 74a and the outer portion of thespring 106 is anchored to the housing 107 as is schematically shown inFIGURE 4.'

As may be further seen in FIGURE 4, spring 106 is so constructed as tobias. disk 104 in a clockwise direction to bring protmsion 108 intoengagement with a fixed stop member 109. In a like manner, spring 105biases gear 74a in a counterclockwise direction withrespect to disk 104through stop 110 on gear 74a and stop 111 on disk 104. Therefore,extension 110 which engages protrusion 11 1 of disk 104, biases disk 104in a clockwise direction.

It is seen therefore, that as gear 74a is free to rotate,

' 108. It is to be noted that no action takes place with respect tospring 106. If in this position the power is interrupted, the addedtension in spring 105 will cause gear 74a to return to the position inwhich the stop 110 on gear 74a comes into contact with the stop 1 11 ondisk 104. At this point, a closed loop is formed and the tension of bothsprings no longer acts on the gear train.

In like manner, if gear 74a rotates counterclockwise, stop 110 picks updisk 104 by means of the stop pin 111 on disk 104. This rotation willcause the spring 106 to wind up such that if the power is interrupted,the spring 106 will cause the disk 104 and the gear 74a to'rotateclockwise until the stop lip 108 engages the fixed stop member 109.

The operation of the system of FIGURE 1 may now be seen in conjunctionwith the schematic diagram of FIGURE 2 which schematically shows theelectrical correcting system as being comprised of two independentcomponents, the first of which is an instrument error corrector and thesecond a static error corrector.

It is first noted that either of thesecorrection systems may be usedindividually or, if desired, may be used together as is shown in thefigure. The block labeled indicator is a schematic representation of theindicating device of FIGURE 1 and similar components have been givensimilar numerals.

Thus, in FIGURE 2 the frame 47 has pointer 39 and Synchrotel 61 mountedthereon. The mechanical difierential 67 which effects rotation of thedrum 47 is shown as being operable through the adjustment knob 519'which is also connected to the barometric counter 48 and through thespring return mechanism 73 which is connected to the motor 72 and thecorrection synchro 74.

Since the indicating instrument 47 has certain inherent errors in viewof its mechanical construction, an instrument error corrector which maybe remotely positioned is provided which has a correcting cam 77 whichcould have a continuous cam surface or, if desired, can be provided witha plurality of adjustable cam points as-is seen in the figure. Thecorrecting cam 77 is positioned by the synchro type servo-mechanismincluding motor 78, Synchrotel 61, amplifying device 79 and synchro 86.Thus, the Synchrotel 61 which may be geared to rotate a little less than360 for full range operation will position correcting cam 77 inaccordance with the pointer position which is'a function of the outputof Synchrotel 61. This in turn positions the cam roller 81 which variesthe position of the rotor of the instrument error synchro 82.

In the event that the instrument error corrector is being used alone,then the instrument error synchro 82 would be connected to thecorrection synchro 74 of the indicator and motor 72 would be energizedthrough the amplifier 83 to rotate the frame 47 of theindicatinginstrument mechanism until correction synchro 74 is rotated to aposition given by the instrument error synchro 82.

In the event of a failure of electrical power which operates the variouselectrical components of the system, it is now understood that thespring return mechanism 73 will return the frame 47 to its uncompensatedposition so that the instrument reading may continue to function on apurely mechanical basis.

If it is now desired to compensate the indicator of FIGURE 2 for staticerror correction, the static error corrector of FIGURE 2 may beconnected as shown.

The static error corrector will, as has been previously set forth,correct for Mach number and for local angle of attack. The static errorcorrecting system may be remotely connected at any convenient point inthe aircraft, as may the instrument error corrector, and is comprised ofan angle of attack synchro 84 which is connected to a sensor of angle ofattack and a Mach synchro 85 which is connected to a sensor of Machnumber of the aircraft. Each synchro 84 and 85operates a systemincluding synchros 86 and 87, respectively, amplifiers 88 and 89,respectively, and motors 90 and 91, respectively, for controlling theposition of a three dimensional cam surface 92.

More specifically, the motor 91 controls the longitudinal position ofthe three dimensional cam surface 92 while motor 90 controls the angularposition of cam surface 92. Hence, the coordinated positioning of camsurface 92 by motors 90 and 91 will be imparted to the cam follower 93to position the shaft 94 of the differential synchro 95 through themultiplying lever mechanism including levers 96 and 97.

Therefore, the motor 72 will be energized until the positions ofsynchros 74, 82 and 95 satisfy one another so as to position frame 47 inaccordance with the requirements of the instrument error corrector andthe static error corrector. Here again, upon a failure of electricalpower, the return mechanism 73 will position the frame 47 in the zerocompensation position so as to allow continued mechanical operation ofthe instrument.

FIGURE 6 shows a modification of the system of FIG- 6 URE 2 when onlythe instrument error corrector is used. In the system of FIGURE 6 it isseen that the corrector cam 77 is placed within the indicator case whilethe amplifiers 79 and 83 are placed in a second remotely positionablehousing.

As the device is subjected to an increase in altitude, the rotor ofSynchrotel 61 is rotated proportionally. The stator of Synchrotel 61 ismaintained at electrical zero with the rotor of motor 78 which at thesame time positions cam 77 which positions the stator of synchro 82accordingly. Synchro 82 is also maintained at electrical Zero by motor72 which positions the rotor of synchro 82 to correspond with its statorposition and at the same time positions the mechanism 47 to achieve acorrection of the reading indicated by pointer 39.

FIGURE 3 shows a second embodiment of my novel invention in which theelectrical system functions to completely determine the pointer readingwith high accuracy, while the position of Synchrotel 61 which rotates asa function of the rotation of pointer 39 feeds this information back tocomputer 100 so that the computer will have a basis for rotating theframe of mechanism 47.

In the event of a failure in the electrical system, a return mechanismwill return the frame to a non-compensated position, and the instrumentwill continue to function as a purely mechanical instrument as was thecase of the system of FIGURE 2.

Thus, as may be seen in FIGURE 3, the indicator is operated from amaster air data computer which positions a synchro 101 in accordancewith its calculated value.

At any given measured value, such as altitude when the device is used asan altimeter, the rotor of Synchrotel 61 will be positioned inaccordance with the mechanical reading of the altimeter mechanism 47.Any error between the Synchrotel 61 and the central air data computersynchro 101 will be amplified by amplifier 102 to cause motor-'72 torotate the mechanism 47 and its associated pointer 39 to the indicatedvalue of the central air data computer.

If power is lost, the return mechanism 73 will return mechanism 47 toits uncompensated position and the indicator will continue to functionas a mechanical system.

When the Synchrotel 61 performs more than one full rotation for theplurality of rotations of pointer 39 over a full altitude range, theinstrument within frame 47 0perates to give a coarse positioning ofpointer 39 within a range of rotation of the Synchrotel while the airdata computer provides the accurate positioning within the coarse range.

In the foregoing, the invention has been described solely inconnectionwith specific illustrative embodiments thereof. Since many variationsand modifications of the invention will now be obvious to those skilledin the art, I prefer to be bound not by the specific disclosures hereincontained but only by the appended claims.

I claim:

I. A spring return mechanism for biasing a rotatable member to apredetermined angular position; said spring return mechanism comprisinga first spring, a second spring, and a rotatable spring carrier; a firstportion of each of said first and second springs being connected to saidspring carrier; a second portion of said first spring being connected tosaid rotatable member; a second portion of. said second spring beingconnected to a relatively stationary member; a stop means on saidrelatively stationary member; said spring carrier being engageable bysaid stop means at a predetermined angular position to prevent rotationof said spring carrier in a first direction beyond said predeterminedangular position; said second spring biasing said spring carrier forrotation in said first direction; an engaging means; said engaging meansremovably engaging said rotatable member and said spring carrier whensaid rotatable member is rotated in a direction opposite said firstdirection to rotate said spring caraoaaeeo Z rierin a direction oppositesaid first directiomsaid first spring biasing said engaging meanstowards engagement with saidspring carrier.

2. spring return mechanism for biasing a rotatable member to apredetermined angular position; said spring return mechanism comprisinga first spring, a second spring, and a rotatable spring carrier; a firstportion of each of said first and second springs being connected to saidspring carrier; a second portion of said first spring being connected tosaid rotatable member; a second portion of said second spring beingconnected to a relatively stationary member; a stop means on saidrelatively stationary member; said spring carrier being engageable bysaid stop means ata predetermined angular position to prevent rotationof said spring carrier in a first direction beyond said predeterminedangular position; said second spring biasing said spring carrier forrotation in said first direction; an engaging means; said engaging meansremovably engaging said rotatable member and said spring carrier whensaid rotatable member is rotated in a direction opposite said firstdirection to rotate said spring carrier in a direction opposite saidfirst direction; said first spring biasing said engagingmeans towardsengagement withsaid spring carrer; said second spring normally maintaining said spring carrier in said predetermined position.

3. A spring return mechanism for biasing a rotatable member to apredetermined angular position; said spring return mechanism comprisinga first spring, a second spring, and a rotatable spring carrier; a firstportion of each of said first and second springs being connected to saidspring carrier; a second portion of said first spring being connected tosaid rotatable member; a second portion of said second spring beingconnected to a relatively stationary member; a stop means on saidrelatively stationary member; said spring carrier being engageable bysaid stop means at a predetermined angular position to with said springcarrier; said second spring normally biasing said spring carrier in saidpredetermined position; said first spring maintaining said rotatablemember engaged with said spring carrier through said engaging meanswhereby predetermined angular position of said rotatable member isdetermined by the position of said stop means.

4. A spring return mechanism for biasing a rotatable member to-apredetermined angular position; said spring return mechanism comprisinga first spring, a second spring, and a rotatable spring carrier; a firstportion of each of said first and second springs being connected to saidspn'ngcarrier; a second portion of said first spring being connected tosaid rotatable member; a second portion of said second spring beingconnected to a relatively stationary member; a stop means on saidrelatively stationary member; said spring carrier being engageable bysaid stop means at a predetermined angular position to prevent rotationof said spring carrier in a first direction beyond said predeterminedangular position; said second spring biasing said. spring carrier forrotation in said first direction; an engaging means; said engaging meansremovably engaging said rotatable member and said spring carrier whensaid rotatable member is rotated in a direction opposite said firstdirection to rotate said spring carrier in a direction opposite saidfirst direction; said first spring biasing said engaging means towardsengagement with said spring carrier; said first and second spring beingwound in a spiral.

5. A spring return mechanism for biasing a rotatable 8 member to apredetermined angular position; said spring return mechanism comprisinga first spring, a second spring, and :a rotatable spring carrier; afirst portion of each of said first and second springs being connectedto said spring carrier; a second portion of said first spring beingconnected to said rotatable member; a second portion of said secondspring being connected to a. relatively stationary member; a stop meanson said relatively stationary member; said spring carrier beingengageable by said stop means at a predetermined angular position toprevent rotation of said spring carrier in a first direction beyond saidpredetermined angular position; said second spring bias-ing said springcarrier for rotation in said first direction; an engaging means; saidengaging means removably engaging said rotatable member and said springcarrier when said rotatable member is rotated in a direction oppositesaid first direction to rotate said spring carrier in a directionopposite said first direction; said first spring biasing sa-id engagingmeans towards engagement with said spring carrier; said second springnormally biasing said spring carrier in said predetermined position;

said first spring maintaining said rotatable member engaged with saidspring carrier through said engaging means whereby said predeterminedangular position of said rotatable member is determined by thepositionof said stop means; said first and second spring being wound in aspiral.

6. A spring return mechanism -for biasing a rotatable member to apredetermined angular position; said spring return mechanism comprisinga first spring, :a second spring, and a rotatable spring carrier; afirst portion of each of said first and second springs being connectedto said spring carrier; a second portion of said first spring beingconnected to said rotatable member; a second portion of said secondspring being connected to a relatively stationary member; a stop meanson said relatively stationary member; said spring carrier beingengageable by said stop means at a predetermined angular position toprevent rotation of said spring carrier in a first direction beyond saidpredetermined angular position; said second spring biasing said springcarrier for rotation in said first direction; an engaging means; saidengaging means removably engaging said rotatable member and said springcarrier when said rotatable member is rotated in a direction oppositesaid first direction to rotate said spring carrier in a directionopposite said first direction; said first spring biasing said engagingmeans towards engagement with said spring carrier; said spring carrierbeing a disc.

7. A spring return mechanism for biasing a rotatable member to apredetermined angular position; said spring return mechanism comprisinga first spring, a second spring, and a rotatable spring carrier; a firstportion oi each of said first and second springs being connected to saidspring carrier; a second portion of said first spring being connected tosaid rotatable member; a second portion of said second spring beingconnected to a relatively stationary member; a stop means on saidrelatively stationary member; said spring carrier being engageable bysaid stop means at a predetermined angular position to prevent rotationof said spring carrier in a first direction beyond said predeterminedangular position; said second spring biasing said spring carrier forrotation in said first direction; an engaging means; said engaging meansremovably engaging said rotatable member and said spring carrier whensaid rotatable member-is rotated in a direction opposite said firstdirection to rotate said spring carrier in a direction opposite saidfirst direction; said first spring biasing said engaging means towardsengagement with said spring carrier; said second spring normally biasingsaid spring carrier in said predetermined position; said first springmaintaining said rotatable member engaged With said spring carrierthrough said engaging means whereby said predetermined angular positionof said rotatable member is determined by the position of 9 said stopmeans; said first and second spring being wound in a spiral; said springcarrier being a disc.

8. A spring return mechanism for biasing a rotatable member to apredetermined angular position; said spring return mechanism comprisinga first spring, a second spring, and a rotatable spring carrier; a firstportion of each of said first and second springs being connected to saidspring carrier; at second portion of said first spring being connectedto said rotatable member; a second portion of said second spring beingconnected to a relatively stationary member; a stop means on saidrelatively stationary member; said spring carrier being engageable bysaid stop means at a predetermined angular position to prevent rotationof said spring carrier in a first direction beyond said predeterminedangular position; said second spring biasing said spring carrier forrotation in said first 10 direction; an engaging means; said engagingmeans removably engaging said rotatable member and said spring carrierwhen said rotatable member is rotated in a direction opposite sa-idfirst direction to rotate said spring carrier in a direction oppositesaid first direction; said first spring biasing said engaging meanstowards engagement with said spring carrier; said spring carrier being adisc; said rotatable member having an extending centrally located shaft;said spring carrier disc being mounted on 10 said shaft and beingrotatable with respect thereto.

References Cited in the file of this patent UNITED STATES PATENTSTrekell Nov. 22, 1955

